(1) Field of the Invention
The present invention relates to a multi-bit-rate optical communication method, an optical network unit, and an optical line terminal and, for example, relates to a technology suitable for use in a system in which a passive optical network (PON) is adopted.
(2) Description of the Related Art
At present, as a subscriber optical fiber network system intended for subscriber homes such as ordinary households, systems that connect an optical line terminal (office) provided at a central office installed, for example, in a telephone central office and optical network units installed in a plurality of subscriber homes via an optical fiber are known. Among them, a configuration is called a PON system in which an optical fiber used for input/output of an optical data signal from/to a central office is branched into a plurality of optical fibers by a power splitter, which is a passive element, and an optical network unit of each subscriber home is connected to the branch destination.
The PON system has been commercially in use as a system capable of transmitting and receiving data at high speed between an optical line terminal and a plurality of subscriber homes.
As a form of communication network using the PON system, for example, a system configuration shown in FIG. 18 can be mentioned.
A PON system 100 shown in FIG. 18 comprises an optical line terminal (OLT) 300, N units of optical network unit (ONU) 200-1 to 200-N (denoted simply as ONU 200 when not distinguished) corresponding to N (N is an integer equal to or greater than 2) subscribers #1 to #N, a power splitter 400, an optical fiber 500 connecting the OLT 300 and the power splitter 400, and optical fibers 600-1 to 600-N (denoted simply as an optical fiber 600 when not distinguished) connecting the power splitter 400 and each ONU 200-1 to 200-N respectively.
In the PON system 100, to perform information delivery and the like, the OLT 300 is an apparatus equipped with a required communication control function of converting an electric signal into an optical signal to transmit the optical signal to an ONU 200 side in a predetermined downstream frame format, converting data transmitted from the ONU 200 as an optical signal in a predetermined upstream frame format into an electric signal or the like.
The optical fiber 500 connected to the OLT 300 is branched by the power splitter 400 provided in midstream of the transmission line, and a branched optical fiber 600 is drawn into each individual subscriber home to be connected to each ONU 200.
The ONU 200 is an apparatus that performs communication control such as communication with the OLT 300 and conversion between an optical signal and an electric signal.
Here, one optical fiber 500 is used for upstream and downstream data transmission between the OLT 300 and the power splitter 400 bi-directionally by wavelength division multiplexing (WDM). Incidentally, the direction from the OLT 300 to the ONU 200 is downstream, and that from the ONU 200 to the OLT 300 is upstream.
For example, a downstream frame from the OLT 300 to the ONU 200 is transmitted as an optical signal in a 1.49 μm band at a single bit rate A by time division multiplexing (TDM), and the ONU 200 detects frame synchronization information and management information in the downstream frame to extract, based on this information, data in time slot individually allocated in advance. Meanwhile, an upstream frame from the ONU 200 to the OLT 300 is transmitted from the ONU 200 with timing provided by the OLT 300. That is, an upstream frame from each ONU 200 is transmitted with timings to avoid collision as an optical signal in a 1.31 μm band by time division multiple access (TDMA) system.
The power splitter 400 plays a role of distributing (power branching) a downstream frame from one optical fiber 500 into a plurality of optical fibers 600 and aggregating (multiplexing) upstream frames from a plurality of optical fibers 600 into one optical fiber 500.
By the way, the PON system 100 has generally been studied for use to provide a low-speed service as low as a telephone service. However, as demand for a high-speed communication service such as a video phone service and a video conferencing service that are faster than the telephone service has increased in recent years, provision of low-bit-rate data (low-speed service such as the telephone service) and high-bit-rate data (high-speed service such as the video phone service and video conferencing service) by one PON system 100 has been desired.
For example, in Patent Document 1 shown below, a method of increasing service capacity in a point to multipoint optical transmission system that realizes an increase in service capacity by using a multi-rate burst circuit that generates a signal of different rate for each time slot is disclosed.                [Patent Document 1] Japanese Patent Laid-Open (Kokai) No. HEI 8-8954        
According to the above-described technology, to provide low-bit-rate data and high-bit-rate data by one PON system, expansion, in addition to a low-speed service, to a high-speed service is enabled by equipping every ONU 200 with a multi-rate burst circuit in advance and assigning a different transmission speed (bit rate) to part of time slots allocated to one ONU 200.
However, since in an existing system, for example, such as a G-PON (Gigabit-capable Passive Optical Network) being standardized by ITU (International Telecommunication Union), a method in which the bit rate is changed for each service in the physical layer to be transmitted is not adopted, there is a problem that the method described in Patent Document 1 is not compatible with existing systems.
Here, in the aforementioned Patent Document 1, compatibility with existing systems as described above is neither considered nor suggested.
Moreover, since the technology described in Patent Document 1 requires preparation (equipment) of a multi-rate burst circuit in every ONU 200 in advance, a subscriber who wants no high-speed service must also upgrade (equipment of a multi-rate burst circuit and so on) an ONU, posing a problem of complicated apparatus upgrading and cost burden thereof.